Plaque can develop in a patient's cardiovascular system. The plaque can be quite extensive and occlude a substantial length of the vessel. Additionally, the plaque may be inflamed and unstable, such plaque being subject to rupture, erosion or ulceration which can cause the patient to experience a myocardial infarction, thrombosis or other traumatic and unwanted effects. Furthermore, relative blood viscosity rises and aggregation of platelets increases with temperature increases (Dintefass L. Rheology of Blood in Diagnostic and Preventive Medicine. London, UK: Butterworths; 1976;66–74). Previous ex vivo studies have shown that there is indeed thermal heterogeneity in human carotid atherosclerotic plaques (Casscells W, Hathorn B, David M, Krabach T, Vaughn W K, McAllister H A, Bearman G, Willerson J T. Thermal detection of cellular infiltrates in living atherosclerotic plaques: possible implications for plaque rupture and thrombosis. Lancet. 1996;347:1447–1449).
An example of a vascular catheter, termed a thermography catheter, is described in our co-pending International patent application number PCT/EP01/04401. The distal tip of the thermography catheter incorporates four sensors mounted circumferentially about a central lumen on resiliently biased projections. The sensors are NTC thermistors that are attached to the end of each projection by bonding with an thermally conducting epoxy glue. Each thermistor is connected to an insulated bifilar wire that provides an electrical connection with the proximal end of the device (not shown). The projections are made of NiTinol and take on the deployed configuration automatically due to their memory properties. The projections are mounted on the central lumen and sandwiched between the central lumen and an intermediate lumen. The point at which the projections meet the central/intermediate lumen terminus is sealed so that the components located between the central and intermediate lumen are electrically isolated from the patient. The thermography catheter is mounted on an angioplasty guide wire which runs through the central lumen and a guide member which defines the tip of the thermography catheter.
In use, the apparatus may be actuated between a deployed configuration and a retracted configuration. In the retracted configuration, a sheath encompasses the projections so that they are constrained to lie parallel to the longitudinal axis of the catheter and therefore cannot take up a deployed position. To adopt the deployed configuration, the sheath is withdrawn away from the extreme distal tip i.e., away from the guide member, towards the proximal section, to expose the projections. When the sheath is withdrawn the resiliently biased projections take up the deployed configuration. It should be noted that the sheath is controlled from the proximal end of the apparatus.
It is important to maintain electrical isolation between the electrical components and the patient. However, it has been found that the original design, described in International Patent Application No. PCT/EP01/04401, suffers from a form of fatigue (strain) failure caused by repeated shortening and lengthening of the electrical wires as the respective projections are deployed and retracted. Repeated compression and tensioning of the wire can cause a failure along the length of the wire and at the electrical connection to the thermistors and/or the sealed terminus between the central lumen and the intermediate lumen, causing an electrical short circuit in use.